1. Technical Field
The present invention relates to a method and a device for carrying out the measurement of a substrate such as glucose included in a specimen.
2. Background Art
There is a conventionally known method called an electrode method, which is a method for measuring a concentration of a substrate such as glucose. According to the method, information correlating with a substrate concentration in a specimen is outputted to an electrode in contact with the specimen, and the substrate concentration is calculated based on the output. An enzyme electrode is conventionally used as the electrode. A conventionally known example of the enzyme electrode has a structure where an enzyme-immobilized film and a substrate selectively permeable film are multilayered on a surface of the electrode.
An enzyme electrode does not have a constant sensitivity. The sensitivity is liable to be degraded depending on environments where it is used, or by a repeated use of the enzyme electrode. The factors causing the degradation of the sensitivity are, for example, alternation of an activity of the enzyme due to temperature changes and deactivation of the enzyme, deterioration of substrate permeability in the substrate selectively permeable film, and oxidization of the electrode surface. The enzyme electrode is thus deteriorated as it is more frequently used and needs to be replaced after being used over a definite period, and calibration is a necessary measure to deal with the sensitivity variation before the replacement of the enzyme electrode.
In an automatic measuring device, for example, the calibration is performed at the startup of the device, and is generally performed at certain time intervals, or per a certain number of measurements in a case where the device is uninterruptedly used over an extended period of time. The sensitivity of the enzyme electrode is variable owing to the various factors described above. In addition to that, the sensitivity may be changeable during the calibration, and it is difficult to predict or grasp what factor causes the sensitivity of the enzyme electrode to change. Therefore, measured values obtained immediately after the calibration can be very reliable, however, it is not necessarily a case that the reliability of measured values obtained between the calibrations is equally high.
In the case of measuring a substrate concentration using an enzyme electrode placed in a flow cell, for example, a base current Rb outputted from the enzyme electrode substantially stays at a constant level as illustrated in FIG. 7A when the sensitivity of the enzyme electrode remains unchanged, and the output from the enzyme electrode is increased when a specimen is introduced. On the contrary, the base output is lowered as illustrated in FIG. 7B under such circumstances that the sensitivity of the enzyme electrode may be degraded.
To calculate the substrate concentration, the output from the enzyme electrode before the supply of the specimen to the enzyme electrode (T1) is sampled as a base output Rb, and the output from the enzyme electrode when a predetermined amount of time passed after the supply of the specimen to the enzyme electrode (T2) is sampled as an output for measurement Re, and a difference between the output for measurement Re and the base output Rb is obtained, so that the substrate concentration is calculated based on the difference. More specifically describing the calculation, the base output when the output for measurement Re is sampled is used as the base output Rb before the output for measurement Re is sampled on the assumption that the base output (sensitivity of the enzyme electrode) is constant.
Therefore, as illustrated in FIG. 7B, a calculated value of the substrate concentration is different to a true value in a case where a base output Rb2 obtained by sampling the output for measurement Re is different to a base output Rb1 previously measured.
When the enzyme electrode deteriorated over time is replaced, the sensitivity of the enzyme electrode shows a sudden drop immediately after the replacement, and is thereafter stabilized to stay at a certain value. Therefore, it is pointless to perform the calibration before the sensitivity of the enzyme electrode is stabilized after the replacement of the enzyme electrode. Thus, the enzyme electrode after the replacement is not ready for measurements until the sensitivity of the enzyme electrode is stabilized and the calibration is completed.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 59-082020    [Patent Document 2] Japanese Patent No. 3723827